1. Field of the Invention
This invention relates to optical transmission systems, and in particular to optical performance monitoring for wavelength division multiplexed (WDM) and dense WDM (DWDM) networks.
2. Background Art
Performance monitoring (PM) refers to in-service, non-intrusive monitoring of transmission quality (GR-253-CORE, issue 2, December 1995). Performance monitoring provides indications regarding the success of attempts to solve a problem, allows to isolate and resolve the problem before it affects traffic, and to determine the entity responsible with the equipment affecting adversely the traffic quality.
The signal power of an optical channel has proved to be one of the most important factors in controlling the performance of an amplified optical network. Firstly, attenuation or transmission loss determines the maximum transmission distance before signal restoration is necessary. Equally important for D/WDM long reach transmission is the equalization of channels, which is necessary for optimal BER performance. Equalization means that all channel co-propagating along a fiber must have substantially similar optical signal-to-noise ratios at their respective receivers.
A number of mechanisms are responsible for the signal attenuation within optical fibers. For a given type of fiber, there are a number of factors that influence these mechanisms. Thus, signal attenuation depends, inter alia, on the distance of the signal travelling along the fiber, launch power, and the number of the co-propagating channels (wavelengths).
These mechanisms are influenced by the material composition of the transmission medium, the preparation and purification technique, and the waveguide structure. They may be categorized within several major areas which include material absorption, material scattering (linear and non-linear scattering) curve and micro-bend losses, mode coupling radiation losses, and losses due to leaky modes. Material scattering becomes an important problem at high powers and large number of wavelengths propagating along the same span of fiber.
Non-linear material scattering losses become especially important as the bandwidth (number of channels and the rate of the channels) and distance of the information transmitted over the fiber increases. Non-linear scattering causes the optical power from one mode to be transferred in either the forward or backward direction to the same, or other modes, at a different frequency. It depends critically on the optical power density within the fiber and hence it becomes significant above threshold power levels. The most important types of non-linear scattering are stimulated Brillouin and Raman scattering, both of which are usually only observed at high power densities. These scattering mechanisms in fact give optical gain, but with a shift in frequency, thus contributing to attenuation for light transmission at a specific wavelength.
Of interest to this invention is stimulated Raman scattering (SRS). Raman scattering is a non-linear phenomenon attributed to the modulation of light through thermal molecular vibrations within the fiber. SRS results in the energy from short wavelengths being transferred to longer wavelengths, which results in cross-talk between the channels in D/WDM systems. To become significant, the launch power must exceed a given threshold dependent on the medium (fiber).
A very effective way of monitoring performance of an end-to-end transmission link is the ‘analog maintenance-1 (AM-1)’ method used in the Applicant's transmission systems. This method is described in U.S. Pat. No. 5,513,029 (Roberts, issued Apr. 30, 96 and assigned to Nortel Networks Corporation), which is incorporated herein by reference.
AM-1 involves amplitude modulating at the transmitter side each channel with a specific pseudo-random dither signal, the dither having a known modulation depth. For each channel, the receiver estimates the power of the received dither and determines the power received on the respective channel using the known modulation depth and the launch power.
However, application of this method to high-speed D/WDM networks is limited because of complexity of the circuitry needed to detect the dithers for each channel, when an increased number of optical carriers share a relatively narrow portion of the spectrum.
Analog maintenance-2 (AM-2) is used by the Applicant for D/WDM networks with a larger number of channels. This method is the object of U.S. patent application Ser. No. 09/539,706 (Harley et al. filed Mar. 31, 2000 and assigned to Nortel Networks Corporation). The above-identified patent application is incorporated herein by reference.
AM-2 provides for modulating each channel with a distinct pulse (digital) sequence, where the RMS of the modulation is proportional to the average power of the respective channel. The far-end receiver recovers the pulse modulation and determines the RMS of the modulation for each channel. The channel power is then estimated based on the modulation depth (known) and the RMS value for the respective channel.
In general, systems with no more than 20 channels are negligibly affected by the SRS, so that AM-2 performance monitoring method can be used without any SRS compensation. However, at very high speeds and systems having over 20 transmission channels, SRS induces Inaccuracies In the power estimate provided by AM-2. The SRS-Induced inaccuracy can be conceptually expressed in the following equation (EQ1) as:                               AM          error                ∝                  log          ⁡                      (                          1              +                                                                    g                    ′                                    ·                  Δλ                  ·                                      P                    0                                    ·                                                            N                      ch                                        ⁡                                          (                                                                        N                          ch                                                -                        1                                            )                                                        ·                                      N                    sp                                                                    2                  ⁢                  α                                                      )                                              EQ        ⁢                                   ⁢        1            
AMerror is the SRS induced in accuracy (in dB), g′ is the Raman gain (inversely proportional to the effective area of fiber), Δλ is the spread of wavelengths (in nanometers), Po is the launch power, Nch is the number of channels, Nsp is the number of spans, and α is the fiber attenuation.
EQ1 shows that the SRS-induced inaccuracy becomes more significant as the number of wavelengths, the launched power, the number of spans and the wavelength spread increase. It is also to be noted that the inaccuracy increases proportionally with the squared number of wavelengths.
SRS-induced AM-2 power measurement inaccuracy can be addressed using a spectrum analyzer. Optical spectrum analyzers are non-obtrusive and not susceptible to SRS, resulting in a better equalization of complex systems in which SRS is significant. When using a spectrum analyzer, AM-2 method is still very useful, in that it provides wavelength identification (wave ID) and optical reflectometer functionality. On the other hand, the optical spectrum analyzer needs to be tightly integrated into the central network management systems. Furthermore, this is a very expensive piece of equipment.
There is a need to provide a reliable method of measuring the power of a channel in high-speed D/WDM system, which accounts for the characteristics of the transport medium and for the inaccuracy introduced by the elements present in the network.